1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer and the like and to one which uses an electrophotographic recording method, an electrostatic recording method and the like. Particularly, the present invention relates to an image forming apparatus which is capable of continuously feeding recording materials, each holding an unfixed image thereon to fixing means.
2. Description of the Related Art
Conventional image forming apparatuses frequently use as a fixing device a heat-contact type fixing device having high thermal efficiency and safety, and particularly a heat roller type fixing device comprising a pair of rollers.
However, the surface temperature of the fixing roller can be nonuniform because the surface temperature of a passage region of the fixing roller through which recording material passes decreases from the passage of the recording materials while heat is not easily absorbed from a non-passage region of the fixing roller. The temperature nonuniformity occurs when the recording materials are continuously passed through the surface of the fixing roller. In this case, if the temperature is set in view of the passage region, the temperature of the non-passage region is significantly higher.
This phenomenon becomes a problem when the set temperature of the fixing roller is changed, when the size of the recording material is changed or for a while after the surface of the fixing roller is brought into a state permitting image formation.
This problem occurs in the heat roller type, a film heating type which uses a thermal heater and a thin film heater and the like.